I assume that the -EPERM error is at the kernel level, and the application gets a return of -1 with errno = EPERM.  It would be much better to return errno = EINVAL so that the error could be easily distinguished from a real EPERM error where the user doesn't have permission to operate on the file.  Since the kernel call is complaining about being unable to support the requested permissions, EINVAL seems like a much more logical error to me.  But let's leave that aside for now.

I assume it is the fchmod() function that is returning the error (sadly, the strace in the original bug report did not contain the process that was actually failing).  If so, you can avoid the mkstemp failure by not asking rsync to preserve permissions (e.g. --no-p).

So, is the issue that rsync is trying to create a mode 0600 file and the mount options are set to make the mount world-readable/writable?  I'd be loath to make rsync retry the fchmod() call with 0666 unless the user asked for that explicitly.  Or perhaps have an option to just skip the fchmod() on the temp file?

Any no-option ideas that are safe?  Perhaps test if the new temp file has bits set outside the umask, and if so, skip the fchmod()?

You might also try changing your mount options to mount the media as a single user, which I would imagine would make a mode of 0600 acceptable to the kernel.

Changing the mount options and/or rsyncing as root did not help in my original tests. But I probably missed a thing with the options reporting they do not affect the problem (see below).

I did more tests today under the newest Fedora 9 kernel (kernel-2.6.25.6-55.fc9).
You are right about the options. This happens only with -p (or -a which is probably why I missed it before). I actually could successfully sync the vfat partition replacing -a with -rt (because I did not need the other components of -a on a FAT partition, anyway).

This means there is a simple workaround but still either rsync or the kernel should be MHO modified to avoid the -p problem.

i`m getting

neoware:/tmp # mount -o loop vfat.dat /mnt/
neoware:/tmp # rsync -av log /mnt
sending incremental file list
log
rsync: chown "/mnt/.log.X0sasg" failed: Operation not permitted (1)

sent 859 bytes  received 31 bytes  1780.00 bytes/sec
total size is 773  speedup is 0.87
rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1040) [sender=3.0.4]

which is 

[pid 23439] lstat64("log", {st_mode=S_IFREG|0755, st_size=773, ...}) = 0
[pid 23439] lchown32("log", 1000, 100)  = -1 EPERM (Operation not permitted)
[pid 23439] select(4, [3], [1], [1], {60, 0}) = 1 (out [1], left {60, 0})
[pid 23439] write(1, "<\0\0\10rsync: chown \"/mnt/log\" fail"..., 64) = 64
[pid 23439] time(NULL)                  = 1249636822
[pid 23439] time(NULL)                  = 1249636822
[pid 23439] select(4, [3], [1], [1], {60, 0}) = 1 (out [1], left {60, 0})
[pid 23439] write(1, "\4\0\0\7\2p\0\0", 8) = 8
[pid 23439] time(NULL)                  = 1249636822
[pid 23439] munmap(0xb7ae5000, 266240)  = 0
[pid 23439] munmap(0xb7b90000, 135168)  = 0
[pid 23439] select(4, [3], [], NULL, {60, 0} <unfinished ...>
[pid 23438] <... select resumed> )      = 1 (in [5], left {59, 992000})
[pid 23438] read(5, "<\0\0\10", 4)      = 4
[pid 23438] select(6, [5], [], NULL, {60, 0}) = 1 (in [5], left {60, 0})
[pid 23438] read(5, "rsync: chown \"/mnt/log\" failed: "..., 60) = 60
[pid 23438] write(2, "rsync: chown \"/mnt/log\" failed: "..., 59rsync: chown "/mnt/log" failed: Operation
 not permitted (1)) = 59


i`m also wondering, why we get "EPERM". i would expect "EINVAL" here. 


Anyway - rsync tries to do chown on a filesystem where it`s not meant to do that.

-p is invalid here, so is -a - but since -p is hidden in "-a" which is commonly and widely used default - and the problem is, that this is not obvious to the average end user.

does rsync have filesystem capabilities detection ? rdiff-backup handles such things very intelligently, since it checks what features exist on the target filesystem. 

anyway - what about adding a hint to chown/chmod related error messages?
if rsync does not intelligently handle that, at least the user should be given a hint where to look at.

neoware:/tmp # rsync -av log /mnt
sending incremental file list
rsync: chown "/mnt/log" failed: Operation not permitted (1)
HINT: not all filesystems support attributs like owner or acl`s, so you may accidentally transfer permission information where you shouldn`t. (options like -p or -a)

Any update on this?

I believe that this question has been answered...

Using rsync's -a or -p is instructing it to do something that is impossible to do on a vfat (and a few others btw) filesystem.  Therefore rsync fails to do what it was told to do and errors appropriately.

The alternative of having rsync ignore part of the user's instruction based on the fact that it will not work and therefore the user must just not be paying attention is wrong and could easily lead to a security issue.  If someone were to rsync a private file to a vfat mount point with insecure permissions that file could then silently become public on the system.

There are many other rsync flags that will not work on a vfat formatted drive and they should all produce appropriate errors too (assuming they encounter a file that triggers that feature).

The reason that rdiff-backup handles this is that it is specifically a backup program.  A backup of data doesn't have to look exactly like the data.  It could for instance be a tarball on a vfat mount that contains hard links, ownerships, permissions and other things that vfat can't handle.  Rsync otoh, is for synchronizing files and directories.  If the target filesystem can't handle what the source filesystem is storing then rsync can't function.

So, the solution is to use -rt instead of -a.  Note that you will probably need --modify-window as well.